Generally, flow meters having various types are used to measure a flow rate of liquid or gas. Among them, a taper pipe-shaped area flow meter has a simple structure and a degree of accuracy high enough to be used for calibration of a flow meter, but may not generate a measured value as an electric signal, and thus utilization thereof is limited to a field in which a flow rate is visually checked through a field indicator by which the measured value is seen through a gradation. Therefore, the taper pipe-shaped area flow meter may not be used in a flow rate control system for a process control.
In a differential pressure flow meter, a flow path of a fluid is fixed, and a pressure difference measured at up and down streams of a fastener, has a functional relation with a flow rate. However, since an area flow meter has a structure in which a flow path of a fluid is changed according to a flow rate, a pressure difference between upper and lower sides of a float is always constantly maintained. That is, in the area flow meter, since a change in an area of a flow path through which a fluid flows is a function of a flow rate, the change in the area of the fluid may be linearly related to the flow rate when a taper pipe is used.
There are many kinds of taper pipe-shaped area flow meters, and a taper pipe-shaped area flow meter 50 as illustrated in FIG. 1, which is one of the most simple taper pipe-shaped area flow meters, includes a taper transparent glass or plastic pipe 51 in which an inlet 53 and an outlet 54 are formed and a float 52 moves vertically in the pipe. Referring to FIG. 1, when a taper pipe 51 in which a wide upper portion narrows toward a lower portion is vertically installed in the taper pipe-shaped area flow meter 50, the float 52 moves vertically in the pipe, and the float 52 moves upward proportional to a flow rate when a fluid flows from the lower portion to the upper portion in the taper pipe 51. A reason for such a motion is that a flow of the fluid serves as a fastener for the float 52 and a pressure difference occurs at a front and a rear of the float 52. When the float 52 moves upward, an area of the taper pipe 51 increases, a pressure difference decreases, the float 52 balances at a position at which the pressure difference balances a weight of the float 52, and the float stops as illustrated in FIG. 1.
In addition, since a flow cross sectional area of the taper pipe 51 determined according to a position to which the float 52 moved vertically is proportional to a flow rate, the flow rate is measured by detecting a position to which the float 52 moved vertically. For example, in a related art published in the Korean Patent Registration (B1) as Korean Patent Registration No. 10-0650526, a conventional area flow meter in which a float is disposed in a taper pipe including an inlet and an outlet is disclosed.
In addition, a float type area flow meter, which is published in the Patent Publication (A) as Publication No. 2002-0006138, includes a main body having a taper flow path in which a diameter thereof decreases downward, a lower end thereof communicates with an inlet of the flow path, and an upper end thereof communicates with an outlet of the flow path, a float accommodated in the taper flow path of the main body and configured to vertically move according to a flow rate, and a distance measurement unit which is installed above the taper flow path of the main body receives relational data between a distance to the float therefrom and a flow rate, measures a position of the float, calculates a flow rate from the relational data between the distance and the flow rate, and outputs the flow rate, and the distance measurement unit includes a laser or ultrasonic sensor located directly above the taper flow path and configured to measure a distance to the float therefrom and output a measured value as an electric signal.